The present invention relates to a tire with a radial carcass reinforcement and more particularly to a tire intended to be fitted to vehicles that carry heavy loads and drive at sustained speeds, such as, for example, lorries, tractors, trailers or road buses.
In general in tires of the heavy vehicle type, the carcass reinforcement is anchored on either side in the bead region and is radially surmounted by a crown reinforcement made up of at least two layers that are superposed and formed of threads or cords that are parallel within each layer and crossed from one layer to the next, making with the circumferential direction angles of between 10° and 45°. The said working layers, which form the working reinforcement, may be further covered by at least one layer termed a protective layer and formed of reinforcing elements that are advantageously metal and extensible, known as elastic elements. It may also comprise a layer of metal threads or cords with low extensibility making with the circumferential direction an angle of between 45° and 90°, this ply, known as the triangulation ply, being situated radially between the carcass reinforcement and the first so-called working crown ply, formed of threads or cords that are parallel and at angles of at most equal to 45° in terms of absolute value. The triangulation ply makes with at least the said working ply a triangulated reinforcement which, under the various stresses it may experience, undergoes very little deformation, the triangulation ply having the essential role of reacting the transverse compressive loads to which all of the reinforcing elements are subjected in the crown region of the tire.
In the case of tires for “heavy” vehicles, there is usually just one protective layer and its protective elements are, in most cases, oriented in the same direction and at the same angle in terms of absolute value as those of the reinforcing elements of the working layer that is radially outermost and therefore radially adjacent. In the case of tires for construction plant intended to run on somewhat uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially inner protective layer being crossed with the inextensible reinforcing elements of the working layer that is radially outer and adjacent to the said radially inner protective layer.
Cords are said to be inextensible when the said cords have, under a tensile force equal to 10% of the breaking strength, a relative elongation of 0.2% at most.
Cords are said to be elastic when the said cords have, under a tensile force equal to the breaking strength, a relative elongation of at least 3% with a maximum tangent modulus of less than 150 GPa.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The axis of rotation of the tire is the axis about which it revolves in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane or equatorial plane is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. An axial distance is measured in the axial direction. The expression “axially on the inside of, or axially on the outside of” respectively means “of which the axial distance, measured from the equatorial plane, is respectively less than or greater than”.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto. A radial distance is measured in the radial direction. The expression “radially on the inside of, or radially on the outside of” respectively means “of which the radial distance, measured from the axis of rotation of the tire, is respectively less than or greater than”.
Some present-day tires, known as “road” tires, are intended to run at high speed over increasingly long distances, because of improvements to the road network and the expansion of the motorway network worldwide. Although all of the conditions under which such a tire is called upon to run undoubtedly allow an increase in the number of kilometers covered, because the tire wear is lower, this is at the expense of tire, and in particular crown reinforcement, durability.
This is because there are stresses in the crown reinforcement and, more particularly, shear stresses between the crown layers, which, when combined with a not-insignificant increase in operating temperatures at the ends of the axially shortest crown layer, have the effect of causing cracks to appear and spread in the rubber at the said ends. This problem exists in the case of edges of two layers of reinforcing elements, the said layers not necessarily having to be radially adjacent.
In order to improve the endurance of the crown reinforcement of the type of tire being studied, solutions relating to the structure and quality of the layers and/or profiled elements of rubber compounds which are positioned between and/or around the ends of plies and, more particularly, the ends of the axially shortest ply, have already been applied.
Patent FR 1 389 428, in order to increase the resistance to damage of the rubber compounds situated near the edges of the crown reinforcement, recommends the use, in combination with a low-hysteresis tread, of a rubber profiled element covering at least the sides and the marginal edges of the crown reinforcement and made up of a low-hysteresis rubber compound.
Patent FR 2 222 232, in order to avoid separation between crown reinforcement plies, teaches the coating of the ends of the reinforcement in a rubber mat, the Shore A hardness of which differs from that of the tread surmounting the said reinforcement, and is higher than the Shore A hardness of the profiled element of rubber compound positioned between the edges of crown reinforcing plies and carcass reinforcement.
French application FR 2 728 510 proposes positioning, on the one hand, between the carcass reinforcement and the working crown reinforcement ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cords making with the circumferential direction an angle of at least 60° and the axial width of which is at least equal to the axial width of the shortest working crown ply and, on the other hand, between the two working crown plies an additional ply formed of metal elements oriented substantially parallel to the circumferential direction.
French application WO 99/24269 also proposes on each side of the equatorial plane and in the immediate axial continuation of the additional ply of reinforcing elements substantially parallel to the circumferential direction, that the two working crown plies formed of reinforcing elements that are crossed from one ply to the next be coupled over a certain axial distance and then decoupled by profiled elements of rubber compound at least over the remainder of the width common to the said two working plies.
This improvement in the endurance of the tires makes it possible to consider at least the possibility of retreading when the tread has become worn. Specifically, when there is a desire to retread the tire when the tread has worn away, in order to optimize the use of the new tread the tire being retreaded has to be at not too advanced a state of ageing.
Moreover, the tires of such vehicles may suffer damage through the tread, for example if pierced by a nail or a bolt. Such piercings of the tread may not be too damaging particularly if the piercing object is removed quickly, notably by human intervention as soon as the presence of the object is detected.
However, if this presence is not detected and if the object pierces right through the crown reinforcement of the tire as far as the internal cavity of the tire, that leads to a loss of inflation air which seeps out slowly along the object.
Such a loss of inflation air, albeit slowly, leads to a drop in pressure that should alert the driver and allow the object to be detected and removed in order to repair the tire. Once again, if this detection is made sufficiently quickly, damage to the tire may be very limited, if any, without in any way jeopardizing the ability for the tire to be retreaded.
Certain vehicles these days are fitted with automatic tire reinflation devices which reinflate the tires as soon as sensors detect a drop in pressure. This reinflation operation is then usually carried out without the knowledge of the driver. If such an operation comes about in the case of a puncture as set out hereinabove, it then becomes possible that the tire will continue to be driven on with a piercing object in it for a relatively long period of time that for example allows the inflation air to reach the rubber compounds of which the tire is made up between the cavity of the tire and the exterior surface of the tire tread.
The object, which may then not be detected until the tire reaches the retreading stage, will then lead to the decision being taken not to go ahead with the said retreading, because the elements of which the tire is made up have been subjected to oxidation for an unknown period of time. Such oxidation may in fact have led to premature ageing of the tire, notably encouraging the spread of cracks within the rubber compounds, the said cracks having been initiated when the object pierced the tire.